The Vietnamese "are distorting the facts, conflating right and wrong on the global stage, blackening China and making unreasonable accusations," Hong said.

The standoff is part of China pushing back against a bigger U.S. presence in the Pacific, according to Hillary Mann Leverett, a foreign policy expert and senior adjunct at American University.

"It really puts Vietnam in the forefront of the battle essentially between the U.S. and China over who's going to be able to exert influence today and for the long term in that critically important part of the world," said Leverett, who has served in the State Department.

China’s message is meant for all of Beijing's territorial rivals, said New South Wales University political professor Carl Thayer.

He said the core states in the Association of Southeast Asian Nations, “not just Philippines and Vietnam, are going to be highly concerned, because it's not just an oil rig, it's the military muscle that goes with it, and every state will feel vulnerable to China."

President Xi Jinping said China always works to resolve peacefully issues of maritime sovereignty and questions Asian nations that seek alliances against neighbors.

"We should respect and be considerate of each country's safety concerns,” he said. “Countries that strengthen military alliances against a third party do not benefit regional security."

Leverett said Beijing is trying to undermine the so-called U.S. pivot based on an assumption that Washington will not fight.

"The idea for China is really to give the United States a choice that it can't win,” Leverett said. “Either it has to fight China over islands it doesn't care about or not back up its allies.”

That “sends a very strong signal to those allies that they don't have the United States to rely on, the pivot's not for real, the one and only neighbor they're going to have to deal with now and forever is China," Leverett added.

Washington said it would support legal action by Hanoi against the oil rig as part of what it says is U.S. "national interest in maintenance of peace and stability" in the South China Sea.